One of the problems in the manufacture of fiber optic cables has been to minimize or eliminate the "microbending" of glass fibers in the finished cable. Microbending is a phenomenon that occurs when radial pressure is applied to the glass surface, forcing it to conform somewhat to the surface roughness of a contacting member. These small amplitude, random bends cause the signal to attenuate. Commonly, such attenuation amounts to a few decibels per kilometer, but it can be as high as a few hundred decibels per kilometer.
Some of the common causes of microbending include tension resulting from winding the fiber or cable on a reel; tension resulting from helical stranding; bending of the cable during installation; pinching the fibers between other cable components; and differential coefficients of thermal expansion which cause tensions and/or compression with resultant radial stresses.
Most cable designs, including telephone, power, and fiber optic cables employ the concept of "stranding" or "oscillating" of individual components to allow for flexibility during manufacture and installation. When a cable is bent into an arc, the portion of the cable on one side of the neutral axis is subjected to compressive forces, and the opposite side is subjected to tensile forces, tending to cause the components to migrate from the compression side to the tensioned side.
In the case of metallic and other conventional cable components, such as wire, these forces are generally not severe enough to cause detrimental effects; namely, elongation, diameter reduction, breakage, etc. In the case of glass fibers, however, these tensile and compressive forces do cause radial pressure on the buffered fiber, resulting in microbending and increased signal attenuation. In more severe cases, fiber breakage can occur. This invention solves these problems by providing a grooved carrier for the glass fiber which is designed to absorb all the forces resulting from bending and flexing of the cable in the process of manufacture and in the finished cable. The carrier also protects the fiber optic element from crushing or pinching. When the stranded (or oscillated) cable is flexed, there is a relative movement of the carrier with respect to other cable components, but the resulting stresses are felt by the carrier; and there is no relative movement between the carrier and the fiber that is placed within the carrier groove. Therefore, no fiber stresses result. Thus the invention provides a construction in which the full benefits of stranding can be achieved without imparting mechanical stresses within the glass fiber.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.